1. Field of the Invention
The present invention relates to an image forming apparatus and a control method therefor that are able to correct skew and misalignment of a sheet conveyed along a conveyance path.
2. Description of the Related Art
When a sheet (transfer material) skews or misaligns during conveyance of the sheet in an image forming apparatus, a conveyance jam (sheet jam) and a sheet handling error with respect to an accessory device will arise. Skew or misalignment of a sheet decreases an accuracy of alignment between the sheet and an image. For example, a position of the image transferred is misaligned with respect to the sheet. Therefore, the image forming apparatus is provided with a mechanism that corrects skew and misalignment of a sheet at just before a position at which an image is transferred to the sheet in order to improve the accuracy of alignment between the sheet and the image.
There are various methods to correct skew and misalignment. For example, there are a diagonal feeding registration method, a no-contact method, etc.
The diagonal feeding registration method uses a skew roller that is arranged to be inclined by a predetermined angle with respect to a conveyance direction, and a contact reference member with which a side edge of a sheet contacts. A sheet is conveyed as a side edge of the sheet contacts with the contact reference member by a conveyance component in the direction perpendicular to the conveyance direction caused by the diagonal feeding roller. Accordingly, a skew correction of the sheet is performed without stopping the conveyance of the sheet. Therefore, the diagonal feeding registration method is advantageous to improve productivity of the image forming apparatus. Since the method that performs skew correction by contacting the side edge of the sheet with the contact reference member is simple, it has an advantage of not needing complicated control.
Many image forming apparatuses adopt configurations to turn upside down a sheet by switching back the sheet after an image is formed on one surface of the sheet when forming images on the both surfaces of the sheet. When using the diagonal feeding registration method in the image forming apparatus that adopts such a configuration, the sheet reversed by a switchback turnover is conveyed while the side edge contacts with the contact reference member similarly. Therefore, a reference of the skew correction to the sheet reversed by the switchback turnover is identical to that to the sheet before the switchback turnover (front and end edges are interchanged by the switchback turnover). Accordingly, there is also an advantage that increases relative registration accuracy in both surfaces of a sheet.
On the other hand, in the diagonal feeding registration method, when a side edge of a sheet contacts with the contact reference member, too strong force that pushes the sheet to the contact reference member may cause a buckling and a loop of the sheet. Then, a guide groove into which a side edge of a sheet is inserted is formed on the contact reference member in order to reduce a buckling and a loop of the sheet by keeping the sheet from the upper and lower sides.
However, since the width of the guide groove of the contact reference member is determined on the basis of the maximum thickness of sheets that can be conveyed, the width is too large to hold a low stiffness sheet (a soft sheet) such as a thin sheet, which causes a buckling or a loop of the sheet. The buckling of a sheet occurs similarly when an amount of curl of a side edge of a sheet is large. Thus, when the buckling occurs, the skew correction is not performed normally and alignment between a sheet and an image cannot be performed in a high precision because of, for example, misalignment of the image with respect to the sheet. As a result, quality of a printed sheet decreases sharply.
A sheet conveyance device with a sheet correction unit that prevents a sheet from buckling when the sheet contacts with the contact reference member is proposed (see Japanese laid-open patent publication (Kokai) No. 2002-356250 (JP2002-356250A)). The sheet correction unit of this sheet conveyance device will be described with reference to FIG. 10. FIG. 10 is a longitudinal sectional view schematically showing a configuration of principal part of the sheet correction unit of the conventional sheet conveyance device.
As shown in FIG. 10, the sheet correction unit of the above-mentioned conveyance device is provided with a side reference guide 80 with which a side edge of a sheet S contacts, a skew roller 81 that aligns the sheet S to the side of the side reference guide 80, and a driven roller 82. The driven roller 82 cooperates with the skew roller 81 to form a nip for holding and conveying a sheet. The side reference guide 80 is arranged adjacent to upper and lower guide members 83 that form a sheet conveyance path. The side reference guide 80 consists of a fixed part 80a and a movable part 80b. The fixed part 80a and the movable part 80b cooperate to form a guide groove into which the side edge of the sheet S is inserted. The movable part 80b always contacts with a cam surface of an adjustment cam 86 that is attached to an output shaft of a motor 85 with a spring 84. The movable part 80b moves in a direction shown by an arrow in FIG. 10 along the fixed part 80a by rotation of the adjustment cam 86. As a result, the width (conveyance gap) of the above-mentioned guide groove is adjusted according to thickness of a sheet etc.
However, since the above-mentioned sheet correction unit adjusts the conveyance gap so that the gap becomes small as the thickness of the sheet decreases, the sheet tends to contact both of the fixed part 80a and the movable part 80b of the guide 80, which increases conveyance friction when the sheet passes through the guide groove. The smaller the conveyance gap is, the smaller the spatial capacity of the guide groove for holding an end edge of a sheet is, when the sheet is received from a conveyance path that is upstream from the guide 80. Accordingly, there is a high possibility that a jam occurs. In order to make the movable part 80b move against the fixed part 80a, a gap is generated between the movable part 80b and the fixed part 80a. When a side edge of the sheet S is inserted into the gap, a jam may occur.
Thus, the method of adjusting the conveyance gap according to a thickness of a sheet tends to generate a jam instead of reducing a buckling.